The art of surveying involves the determination of unknown positions, surfaces or volumes of objects or setting out of known coordinates using angle and distance measurements taken from one or more positions. In order to make these measurements, a surveying device frequently used is a distance measuring instrument with an integrated distance and angular measurement of the type which is called a total station, i.e. with combined electronic, optical and computer techniques. A total station is furthermore provided with a computer or control unit with writable information for measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system. In, for example, WO 2004/057269 by the same applicant, such a total station is described in more detail.
When performing distance measuring or surveying tasks using a distance measuring total station at a work site, naval work site, a construction work site or a mining work site, it is often desirable to track a moving object or measure the distance to an object.
The tracker system of an optical total station normally focuses a light signal from a target located at an object, either a reflection of an incident measuring beam from the target or from an active device on the target, onto a detector. A servo system of the total station moves or turns the station in according to the signal from the sensor.
Thus, the tracker system will only have a limited angle of view and in order to locate or track the target, the tracker system will have to be pointed in a direction close to the target. When the servo system has pointed the total station towards the target, it will be able to “lock onto” the target, which means, inter alia, that the total station can follow or track the targets motion.
This narrow angle of view entails problem, for example, when the target is temporarily lost, i.e. the reflecting signal is lost, when the target for instance is hidden behind an obstacle when the object moves behind the obstacle. In order to regain the lock of the target when the target reappears, the total station needs to estimate the movement of the target, i.e. the object, behind the obstacle. Conventionally, this has been done by turning or moving the total station with the same angular velocity as before the target was lost. However, this in fact corresponds to a target moving along a circle around the total station, which is an uncommon case. In most cases, the target moves along a linear or slightly curved path, which not necessarily is perpendicular to the line of sight of the total station. A target moving with constant speed along a linear path relative the total station will not have a constant angular velocity. The angular velocity will, for example, increase significantly in the vicinity of the total station, especially when the target passes close to the total station.
A similar problem arises in the distance meter, which uses a pulse measurement technique where the exact time a transmitted pulse returns from the target is determined. The distance meter utilizes a narrow sampling window, i.e. a short period of time, which entails that it is able to sample only a short period of time for each returning pulse. Consequently, in order to obtain measurement data from a certain returning pulse, the distance meter must know approximately when it will be returning. Since the time of flight corresponds to the distance to the target this means that the distance meter has to know the approximate distance to the target.
When measuring against an unknown target, the distance meter will move the sampling window and search for the returning pulse, which make take time. Once the target has been found, the distance meter will keep the sampling window locked and will start shifting or moving the sampling window if the target starts moving to compensate for the distance change. If the measuring signal has been obstructed for some reason, for example, due to that the target is hidden temporarily behind an obstacle, the distance meter may loose the target, i.e. the sampling window may not capture the returning. In order to resume the distance measuring procedure, the distance meter have to estimate the distance to the target when it reappears after the obstacle, which may be difficult due to, for example, a non-linear moving velocity of the object. Accordingly, the distance meter may have to search for the returning pulse and move the sampling window, which may take time.
Thus, there is need of an improved and more efficient total station and a method for such a total station for estimating, for example, a position of a moving object.